Sexual reproduction in plants and animals requires the production of gametes. Although many cytological differences exist in the developmental stages of gamete development between the plant and animal kingdoms, several parallels exist. In plants as well as in animals, gamete production is a highly ordered process characterised by transitions of stem cells from one physiological state to the next by mitotic and meiotic divisions. These series of events are directed by multiple changes in gene expression. Several stages in gamete development in several plant and animal species proceed almost without any transcriptional activity. In these stages, previously synthesised mRNAs are translated into products that are essential for further development. This implies that, in these species, post-transcriptional control of gene expression is the leading principle in gamete development. An example of post-transcriptional regulation of gene expression during gamete development is the maturation and germination of the male gametophyte (pollen) in angiosperm plants. Immature pollen consists of a small generative cell and a large vegetative cell which are formed out of microspores through an asymmetric haploid mitotic division (pollen mitosis I). During the subsequent stages in development some species have a second haploid mitosis (pollen mitosis II) resulting in tricellular pollen, a process absent in most species (i.e. bicellular pollen). Maturation of this bi- and tricellular pollen is completed by a range of developmental processes which finally result in a progressive dehydration of the pollen and its transition to dormancy. Maturation of pollen of several plant species is accompanied by an accumulation of large quantities of rRNAs, tRNAs, mRNAs and ribosomes. As soon as the pollen lands on a compatible stigma, an extensive rehydration of the pollen grain occurs leading to a rapid reactivation of the translation machinery which uses the previously accumulated tRNAs, rRNAs and mRNAs. The proteins that are synthesized from these stored products are required for the progamic life stage of the pollen, i.e. germination of the pollen, the subsequent growth of the pollen tube and the second haploid mitosis in case of the bicellular pollen.
Despite the importance of post-transcriptional processes for the regulation of pollen gene expression, little is known about the mechanisms underlying post-transcriptional regulation of pollen gene expression. There is only one study in the art that focuses on the translational regulation of a pollen expressed gene (lat52) (Bate et al., (1996) Plant J., 10(4), 613-623). In this study, the involvement of the 5′ UTR in the pollen specific regulation of translation has been demonstrated. More in general, the rate of translation is influenced by cis-acting elements in mRNAs. E.g. the translation level of mRNA species from different eukaryotic systems is modulated by cis-acting elements in the 5′ UTR, the coding sequence, or the 3′ UTR. These cis-acting elements act by influencing mRNA stability, translation initiation or elongation.
The regulation of the synthesis of the tobacco pollen protein NTP303 takes place at the post-transcriptional level. Transcripts of the ntp303 gene are first detectable after pollen mitosis I and continue to accumulate during pollen maturation and subsequent pollen tube growth (Weterings et al., (1992) Plant Mol. Biol. 18(6), 1101-1111). In contrast, the protein only appears in detectable amounts at the onset of pollen rehydration (Wittink et al., (2000) Sex. Plant Reprod. 12(5), 276-284). Thus, despite the accumulation of its mRNA there is no efficient synthesis of the NTP303 protein during pollen development, which constraint is only relieved at the onset of pollen germination. It is, however, not clear what cis-acting elements in the npt303 mRNA, if any, are responsible for this mechanism of translational regulation. In particular it is not clear whether any such elements could be used to regulate the expression of proteins other than NPT303, such as heterologous proteins.